Routine maintenance is key to keeping equipment operating reliably, and heat transfer systems are no exception. While the fluids in these systems are often described as “low maintenance,” there will come a point when you’ll need to evaluate whether a change is necessary.

Here we’ll focus on small systems (guidelines for larger systems are available here).

Small systems are usually electrically heated. In some cases, they’re standalone portable units, while in others they’re integrated into a larger process, such as an extruder, press, or calendering roll. These systems often include built-in safeguards like pressure relief bypasses and simplified controls, making them easy for most operators to use. They’re designed for quick connection to a process inlet and outlet and are common in applications like plastics processing, die casting, and other industries requiring compact oil heaters.

Because these systems typically use tanks or reservoirs open to the atmosphere, exposure to air makes oxidation the leading cause of fluid breakdown.

Conducting a fluid analysis is the best way to determine the extent of degradation. Once you’ve reviewed the results, here are some things to keep in mind:

If the fluid remains serviceable but shows early signs of degradation, it may still be usable. For cases of oxidation, check tanks, reservoirs, or other air-exposed areas. If they are running hotter than 200°F, try lowering the temperature to slow the oxidation rate.

If thermal degradation is detected, verify that your equipment is functioning properly. Inspect pressure relief bypasses and heaters to ensure they’re working correctly, since these areas often contribute to overheating and breakdown.

When it’s time to replace the fluid, a complete oil change is essential. If old, degraded fluid isn’t fully removed, it will contaminate the fresh charge, shortening its life.

This step can be more challenging in small systems because fluid tends to remain trapped in places like heat exchangers, filter housings, and horizontal lines. Be sure to check these areas for leftover fluid. Using compressed air or nitrogen to blow out the system is usually an effective way to clear residual oil.